1. Field of the Invention
The present invention relates to an eyepiece optical system (finder optical system) provided in a photographing apparatus, such as an SLR (single-lens reflex) camera.
2. Description of Related Art
In an eyepiece optical system of an SLR camera, an object image formed via a photographing lens is formed onto a focal-plane plate (focusing plate). Thereafter, upon the object image that is formed on the focal-plane plate being inverted to produce an erected image via an image inverter, such as a pentagonal prism (hereinafter referred to as a “pentaprism”), etc., the erected image is magnified and displayed via an eyepiece optical lens system (loupe optical system), so that a user can observe the (magnified and erected) object image.
The above-described eyepiece lens system is required to have an adjustable diopter to suit the user's eyesight, is required to ensure a sufficiently long eye relief (the distance between the eyepoint and the apex of the outermost surface on the eyepoint side of the eyepiece optical system), is required to have a high observation magnification, and is required to have a favorable optical quality having a minimal amount of distortion so that the object image (formed through the photographing lens) can be viewed with substantially no distortion.
Patent Literature 1 (Japanese Unexamined Patent Publication No. 2010-237430) discloses an eyepiece optical system which is provided with a negative distortion-correction lens element (field lens), between a focal-plane plate (focusing plate) and an image-erecting prism (pentaprism), for correcting distortion that occurs in a negative lens group positioned closest to the viewing side (eye side) within a loupe optical system.
Patent Literature 2 (Japanese Unexamined Patent Publication No. 2012-242740) discloses an eyepiece optical system which is provided with a positive condenser lens element positioned between a surface (focusing plate) on which an object image is formed, via a photographing lens, and an image-inverter member (prism), for producing an erected image, in order to achieve a high observation magnification.
In Patent Literature 1, since the paraxial refractive power of the distortion-correction lens element is negative, the focal length of the entire eyepiece optical system is longer than the focal length of the loupe optical system, which is provided on the viewing side with respect to the erecting prism (pentaprism/image-inverter). Therefore, there is a problem with the observation magnification being lower than when there is no distortion-correction lens element provided in the eyepiece optical system. Furthermore, although distortion in the eyepiece optical system is maintained at a reduced level by utilizing an aspherical surface on the distortion-correction lens element, the diverging function at the peripheral area of distortion-correction lens element becomes too strong due to the paraxial refractive power of this aspherical surface being negative and having an increasingly stronger negative refractive power with respect to a direction away from the optical axis. Accordingly, since the peripheral portion of a Fresnel lens (functioning as a condenser lens) provided at the incident surface of the focal-plane plate (typically a focusing surface on which the exiting surface forms an object image) is required to have a stronger converging function, an extremely strong positive refractive power is required at this peripheral portion. Asa result, the Fresnel zone inclination angle at the peripheral part of the Fresnel lens becomes too large, thereby making manufacturing thereof difficult.
In Patent Literature 2, by providing a condenser lens element having a positive paraxial refractive power in front of an image-inverter member (prism), the focal length of the entire eyepiece optical system becomes shorter than the focal length of a loupe optical system, which is provided on the viewing side (eye side) of the image-inverter member (prism), thereby increasing the observation magnification of the eyepiece optical system compared to when such a condenser lens element is not provided. However, distortion that occurs in the loupe optical system is not corrected.